1. Field of Invention
The present invention generally relates to an illumination system comprising a radiation source and a fluorescent material comprising a phosphor.
2. Description of Related Art
Recently, various attempts have been made to make white light emitting illumination systems by using light emitting diodes (LEDs) as radiation sources. When generating white light with an arrangement of red, green and blue light emitting diodes, there has been such a problem that white light of the desired tone cannot be generated due to variations in the tone, luminance and other factors of the light emitting diodes.
In order to solve these problems, there have been previously developed various illumination systems, which convert the color of light emitted by light emitting diodes, by means of a fluorescent material comprising a phosphor to provide a visible white light illumination.
Previous illumination systems have been based in particular either on the trichromatic (RGB) approach, i.e. on mixing three colors, namely red, green and blue, in which case the latter component may be provided by a phosphor or by the primary emission of the LED or in a second, simplified solution, on the dichromatic (BY) approach, mixing yellow and blue colors, in which case the yellow component may be provided by a yellow phosphor and the blue component may be provided by the primary emission of blue LED.
In particular, the dichromatic approach as disclosed in, for example, U.S. Pat. No. 5,998,925 uses a blue light emitting diode of InGaN semiconductor combined with a Y3Al5O12:Ce3+ (YAG-Ce3+) phosphor. The YAG-Ce3+ phosphor is coated on the InGaN LED, and a portion of the blue light emitted from the LED is converted to yellow light by the phosphor. Another portion of the blue light from the LED is transmitted through the phosphor. Thus, this system emits both blue light emitted from the LED, and yellow light emitted from the phosphor. The mixture of blue and yellow emission bands are perceived as white light by an observer with a CRI in the middle 80s and a color temperature, Te, that ranges from about 6000 K to about 8000 K.
However, white light LEDs based on the dichromatic approach can only be used to a limited extent for general-purpose illumination, on account of their poor color rendering caused by the absence of red color components.